As Tor-Einar Jarnbjo correctly pointed out (+1), Schengen visas cannot generally be obtained from within France (or, in fact, anywhere in the Schengen area). It's sometimes (but not always) possible to apply for a residence permit from within a Schengen member state. In France, there are some exceptions (and as a PhD student, you might qualify for one) but you generally need to obtain a long-stay visa from a French consulate abroad before applying for a student residence permit (i.e. you can't apply for it if you are visiting the country on a short-stay visa).
Furthermore, Schengen visas (and other French visas) should be obtained from the consulate competent for your usual place of residence. In practice, the requirements vary and you might be able to skirt the rules but in principle it means that merely going to the UK, Turkey or some other place outside the Schengen area is not enough, you also need to be a resident there to apply for a Schengen visa. Otherwise, you might have to go to your last country of residence.
In any case, the relevant authority to contact within France for all immigration matters is the préfecture of your place of residence (in Paris, the préfecture de police).
